The present invention relates to a process and apparatus for continuously producing a high concentration sugar solution. The present apparatus includes a continuously operating centrifuge to which the massecuite is continuously supplied. The centrifuged sugar crystals are contacted by a solvent liquid in such a manner as to enhance the solving action whereupon the solution is removed from the centrifuge.
A centrifuge which may be modified for the present purposes includes a screen basket, which is rotatable about a vertical axis. The screen basket is surrounded by an inner housing providing a collection space for the liquid separated from the sugar crystals. An upper rim of the screening basket extends above the inner housing. The inner housing is surrounded by an outer housing including a cover through which massecuite supply means extend into the basket. Preferably, the massecuite supply means extend down into a distribution cup which is secured to the upper end of a drive shaft for the centrifuging basket.
It is known to produce high purity sugar by guiding a previously purified sugar solution through crystallizing devices, the output of which is the so called massecuite. The massecuite is then separated in centrifuges into a liquid component and into a solid component, the latter constituting the crystallized sugar. Contaminations which might still be present in the starting material are thus retained in the liquid component. However, very small quantities of contaminations also remain in the sugar crystals after the centrifuging. Thus, where it is intended to produce a high purity grade sugar, it is customary to again dissolve the centrifuged sugar crystals, whereupon the solution is again crystallized and subjected to a separation action in a centrifuge Prior to the second crystallization the sugar solution obtained by the second dissolving is passed through suitable filter means.
High concentration and high purity sugar solutions are used for industrial purposes, for example, in the beverage industry. Such sugar solutions are transformed into so called invert sugar in order to prevent the recrystallization. However, before such conversion may be performed, it is necessary to produce the required sugar solution by way of crystallization and again dissolving the crystals and filtering the solution. Heretofore it was customary to employ the batch method where extremely high grades of purity for the sugar solution were desired. However, continuously operating centrifuges have also been employed.
Thus, U.S. Pat. No. 3,730,769, granted May 1, 1973 discloses a method and apparatus wherein a continuously operating centrifuge is employed. The centrifuge of this prior art publication comprises a conical centrifuging basket which is provided at its upper end with a radially extending rim. This rim extends into a so called circular collecting ring pipe arranged coaxially relative to the rotational axis of the centrifuging basket. The circular ring pipe is provided with a slot facing toward the basket so that the basket rim may extend into such slot. The circular ring pipe is supplied with a liquid solvent and an outlet port is operatively connected to the ring pipe at a suitable point for removing the flowable product from the ring pipe. It is the purpose of the ring pipe to again dissolve the sugar crystals which emerge from the centrifuging basket directly into the solvent charged ring pipe. However, practical experience has shown that the ring pipe contains mostly liquid and sugar crystals so that at best only a mash-in is accomplished in this reference. Thus, according to this prior art approach, it is necessary to convey the liquid sugar crystal mixture produced according to German Patent Publication No. 2,025,828 through mixing pumps or through respective solution containers including stirring mechanisms in order to provide a sugar solution which may be filtered.
It appears that in the just mentioned reference there is insufficient time for the sugar crystals to enter into solution in the ring pipe. It is technically not possible to provide the necessary residence time for the sugar crystals in the ring pipe since the latter must not be clogged. Further, where the conditions which influence the solution are the same, the solution itself may only be shortened in time by mechanically influencing the crystal liquid mixture. This is so, because during the dissolving each sugar crystal is enveloped by a coating of a saturated or substantially saturated solution, and it is not possible to mechanically influence such coating inside the just described ring pipe.
A further dissolving is only then possible when the coatings of high sugar concentration are replaced by liquid of low sugar concentration. However, such replacing becomes increasingly more difficult due to the viscosity which also increases with the increasing concentration. Due to these physical facts, it was customary heretofore to produce sugar solution in solution containers provided with respective mechanical stirring devices. The mechanical stirring enhances the exchange or replacement of the envelope layers or coatings of high concentration with such envelope layers or coatings having low sugar concentrations.
In the centrifuge according to German Patent Publication No. 2,025,828 the liquid flows relatively slowly in the collection ring, because a centrifugal effect would be noticeable if the speed becomes too high in the collection ring. Thus, sugar would settle down in the outer range of the collection ring and the liquid flow would not be able to entrain the sugar crystals. However, the speed of flow which is small enough to prevent such sedimentation, is much too small to mechanically influence the solid, liquid mixture so as to noticeably reduce the time duration required for the solution.
In view of the foregoing it will be appreciated that the just described prior art centrifuge discloses features for preventing the so called knot formation or clogging. The clogging or knot formation is almost unavoidable in continuously operating centrifuges from which the sugar is conventionally discharged in a dry state. Such sugar knots or lumps in turn make the subsequent solving of the sugar more difficult. This is avoided by the so called liquid discharge of the sugar from a continuously operating centrifuge, because the solid, liquid mixture may be conveyed relatively easily.
U.S. Pat. No. 2,883,054 discloses a centrifuge in which the sugar discharged from the conical centrifuging drum is sprayed with a liquid in a collecting ring from which the sugar may be rinsed. This reference also mentions that it is possible to dissolve again the sugar in the manner described. However, as mentioned above, this is physically impossible due to the short residence time of the liquid sugar mixture in the collecting ring.
U.S. Pat. No. 3,301,708 discloses another example in which a continuously operating centrifuge comprises a collecting ring surrounding the upper drum rim. The sugar collected in this ring is exposed to a liquid. For this purpose the reference discloses a rotating nozzle body to which the liquid is supplied.
U.S. Pat. No. 3,238,063 describes a continuously operating centrifuge comprising a ring pipe arranged with a relatively large axial spacing from the upper edge of the conical centrifuging basket. The ring pipe is provided with radially outwardly directed nozzle openings for the liquid. In this known centrifuge the sugar discharged over the upper edge of the centrifuging basket impinges upon elastically yielding baffle walls, whereby simultaneously liquid discharging from the nozzles in the ring pipe is applied to the sugar crystals. The purpose of this arrangement is to produce a solid, liquid mixture which may easily be discharged from the centrifuge without the formation of sugar lumps or knots.